Yongxuan Sun

Polarization-independent longitudinal multi-focusing metalens.

A novel multi-focusing metalens in the longitudinal direction has been proposed and investigated based on the equal optical path principle, which is independent on the incident polarizations and can be suitable for both of the linear and circular polarization incidences simultaneously. Here, three novel designing principles: partitioned mode, radial alternating mode and angular alternating mode, have been proposed firstly for constructing different types of the longitudinal multi-focusing metalenses. The performances of the designed metalenses based on the different designed methods have also been analyzed and investigated in detail, and the intensity ratio of the focusing spots can be tuned easily by modulating the numbers of the relative type of nanoantennas, which is significant for the micro-manipulating optics and the multi-imaging technology in the integrated optics.

Broadband, high-efficiency, arbitrary focusing lens by a holographic dielectric meta-reflectarray

In this paper, a metalens based on the dielectric meta-reflectarray consisting of silicon nanorods in combination with a gold ground plane is proposed to realize an arbitrary focusing lens. We have demonstrated that the meta-reflectarray is served as a half-waveplate with near-unity reflectance and over 98% polarization conversion efficiency over a wavelength range from 1.5 to 1.6 μm for circularly polarized light incidences. We have also demonstrated that single spot and four-spaced spots focusing with more than 96% diffraction efficiency over 100 nm bandwidth can be realized by this metalens in the near infrared band just by controlling the reflection phases. The spatial phase distributions of the corresponding designed metalens can be determined via a computer-generated hologram method. Meanwhile, the desired phase can be simply obtained by modulating the orientation of the silicon nanorods. The proposed approach demonstrates a high-performance solution for creating low-cost and lightweight beam-shaping and beam-focusing devices at telecommunication wavelengths.

Plasmonic focusing lens based on single-turn nano-pinholes array.

A miniature, simplified and planar plasmonic lens based on the circular array of nano-pinholes for on-axis beaming has been proposed and investigated systematically in the visible spectrum. Focusing properties of the designed plasmonic lens illuminated under circular polarized (CP) light for different radius of circular ring, filled with different dielectrics, with different numbers of pinholes have been investigated and analyzed in detail by finite element method (FEM). Our simulated results demonstrate such a miniature single-turn structure can also generate a totally centrosymmetric focusing spot under the CP illumination. Besides, by properly manipulating the filled dielectric and incident wavelengths, enhanced transmission, elongated depth of focus have also be realized, which can be used to modulate the transmitting fields effectively. Such a miniature and simplified plasmonic focusing lens can open up a vital path toward fiber-end planar photonic devices for biosensing and imaging.

High-efficiency refractive index sensor based on the metallic nanoslit arrays with gain-assisted materials

Abstract We have designed and investigated a three-band refractive index (RI) sensor in the range of 550–900 nm based on the metal nanoslit array with gain-assisted materials. The underlying mechanism of the three-band and enhanced characteristics of the metal nanoslit array with gain-assisted materials, have also been investigated theoretically and numerically. Three resonant peaks in transmission spectra are deemed to be in different plasmonic resonant modes in the metal nanoslit array, which leads to different responses for the plasmonic sensor. By embedding the structure into the CYTOP with proper gain-assisted materials, the sensing performances can be greatly enhanced due to a dramatic amplification of the extraordinary optical transmission (EOT) resonance by the gain medium. When the gain values reach their corresponding thresholds for the three plasmonic modes, the ultrahigh sensitivities in three bands can be obtained, and especially for the second resonant wavelength (λ2), the FOM=128.1 and FOM* = 39100 can be attained at the gain threshold of k =0.011. Due to these unique features, the designing scheme of the proposed gain-assisted nanoslit sensor could provide a powerful approach to optimize the performance of EOT-based sensors and offer an excellent platform for biological sensing.

Circular polarization analyzer based on an Archimedean nano-pinholes array

A relative broadband circular polarization analyzer based on a single-turn Archimedean nano-pinholes array has been proposed and investigated systematically from visible spectrum to near infrared region. The spiral arrangement of circular nano-pinholes can implement spatially separated fields according to the relationship between the spiral direction of Archimedean structure and chirality of circularly polarized light (CPL). The enhanced-characteristics mechanisms of the single-turn spirally arranged Archimedean pinholes array have been deduced and investigated by the theoretical analysis and numerical simulation in detail. Different from the single operating wavelength of the spiral slit structure, this novel design also shows a relative wide range of the operating wavelengths in the focusing and defocusing effects. The new proposed circular polarization analyzer could find more extensive applications, such as analyzing the physiological properties of chiral molecules based on circular polarizations, full Stokes-parameter polarimetric imaging applications and so on.

Enhanced Forward Scattering of Ellipsoidal Dielectric Nanoparticles

Dielectric nanoparticles can demonstrate a strong forward scattering at visible and near-infrared wavelengths due to the interaction of optically induced electric and magnetic dipolar resonances. For a spherical nanoparticle, the first Kerker’s condition within dipole approximation can be realized, where backward scattering can reach zero. However, for this type of dielectric sphere, maximum forward scattering without backward scattering cannot be realized by modulating the refractive index and particle size of this nanoparticle. In this paper, we have demonstrated that a larger directional forward scattering than the traditional spherical nanoparticle can be obtained by using the ellipsoidal nanoparticle, due to the overlapping electric and magnetic dipolar modes. For the oblate ellipsoid with a determined refractive index, there is an optimum shape for generating the suppressed backward scattering along with the enhanced forward scattering at the resonant wavelength, where the electric and magnetic dipolar modes overlap with each other. For the prolate ellipsoid, there also exist the overlapping electric and magnetic dipolar modes at the resonant wavelength of total scattering, which have much higher forward scattering than those for both oblate ellipsoid and sphere, due to the existence of the higher multipolar modes. Furthermore, we have also demonstrated the realization of the dimensional tailoring in order to make the strong forward scattering shift to the desired wavelength.

Acquiring reflective polarization from arbitrary multi-layer surface based on Monte Carlo simulation

A novel Monte Carlo model is proposed to acquire the reflective polarization information from a rough surface with arbitrary layers and profiles. Based on the micro-facets theory, the local normal vectors can be randomly sampled from the normal vector distribution of each layer. The incident light that propagates inside of the multi-layer media will be traced until being collected after leaving the surface or be ignored due to lacking enough energy. The simulated results (by our proposed theoretical model) agree well with the reported measured data and the analytical models from SCATMECH, which demonstrates the correctness and effectiveness of our model. Based on our model, the effects of the surface layer number, the surface geometry, the incident wavelength and polarization states of incidence on the reflective polarization from multi-layer surfaces have been analyzed in detail, which can be a guide in tasks such as target detection and so on.

Polarization-independent characteristics of the metasurfaces with the symmetrical axis’s orientation angle of 45° or 135°

A series of symmetrical nanoantennas with a symmetrical axis orientation angle of 45° or 135°, which are suitable for both X/Y linear and circular polarizations incidences simultaneously, have been designed and investigated in detail. We have deduced the transmitted matrix of the metasurface structure by rigorous mathematical theory, and found that the essential reason for the polarization-independence characteristics is that there are the same transmitted amplitudes and phases under the incidences of X/Y linear and circular polarization lights due to metasurface structure with the symmetrical axiss orientation angles of 45° or 135°. Based on the V-shaped, C-shaped, U-shaped and elliptical slit nanoantennas, we have verified the proposed theory fully by numerical simulations. The independence of the incident polarizations is very important for the practical applications and developments of the metasurfaces.

Metalens Focusing the Co-/cross-polarized Lights in Longitudinal Direction

Recently, metasurface has attracted lots of attentions because of its great capability in phase engineering for the transmitted cross-polarization light, and many functional optical elements have been designed and investigated. Commonly, the co-polarization and cross-polarization lights will coexist in the transmitted fields. Here, we propose a planar metalens composed of L-shaped nanoholes, which can focus an incident plane wave to two different focal spots in longitudinal direction for the co-polarized and cross-polarized transmitted lights respectively. In our design, the focal length of the transmitted cross-polarized lights can be tuned easily according to Fermat principle. Meanwhile, the focal length of the co-polarized transmitted lights can also be modulated by the ring number of the designed metalens. Because of the polarization-independent characteristic of the L-shaped nanoantenna, the designed planar metalens can also be suitable for both linear and circular polarized incident lights.